1. Field of the Invention
The present invention relates generally to a tin-lead eutectic solder, and, more particularly, to a composition for forming a tin-lead eutectic alloy with controlled composition by electroplating.
2. Description of Related Art
Eutectic and near-eutectic tin-lead solder alloys are used to provide solder joints in a variety of electronic devices. The solder joint provides both an electrical connection and a mechanical link between electronic devices and connectors. In order to improve the solder joint, one or both of the surfaces to be soldered together are sometimes electroplated with a layer of the solder alloy prior to the soldering procedure. This electroplating is accomplished using known electroplating techniques and a solution of a metal salt and a strong acid or base, as described, for example, in "Metal Finishing 1991 Guide Book and Directory", Metal and Plastics Publication Inc., page 272.
In the case of a tin-lead eutectic solder, a typical plating bath solution comprises: 7-8 ounces per gallon (oz/gal) stannous tin, 3-4 oz/gal lead, 3-5 oz/gal fluoroboric acid, and 2-3 oz/gal Peptone.
One problem that arises in the use of such a solution is that the fluoride ion from the fluoroboric acid combines with tin to form insoluble or colloidal products which are not readily removed by filtration and which contaminate the electroplated deposit and requires that the plating bath be replaced about every 6 months. Another problem which arises in the use of the above-noted solution is that the Peptone codeposits in the plated deposit. These effects result in a deposit in which the tin-lead ratio on a single surface varies and causes unreliable solder connections. In addition, the solder discolors and has poor solderability, the solder flow is erratic, the solder has poor cosmetic appearance, and there is a relatively high incidence of touch-up required.
Thus, a need exists for a composition for plating a tin-lead alloy which does not form any unwanted deposits in the plated deposit, and which has a uniform and closely controlled tin-lead composition.